In many industries it is often desirable to form a large ring from various metal or alloy materials. Typically, in some examples, an initial hollow cylindrical ring is first manufactured, for example by forging and/or machining. In the case of some applications for the final ring, such as for example aerospace parts, the desired final rings may have large diameters and/or may be made of very high temperature alloys or superalloys. The initial hollow ring is then expanded to have a larger inner and outer diameter. One way of expanding a ring is by so-called vertical rolling, in which the ring is inserted so it is pinched with its thickness between two driven rollers. One of the rollers is driven and a compressive force is applied between the rollers so that the ring moves through the rollers and is squeezed as it goes around, thus gradually reducing the thickness of the ring while increasing its inner and outer diameters.
In the prior art, the driven roller has been driven by an electric motor. These systems have been found to be very satisfactory for some applications. However, the electric motor drives for the roller typically require gear reduction gearboxes and alignment couplings. For example, in one type of ring mill, a 500 hp motor might be reduced from 1800 RPM to 18 RPM to get a high torque output on the roller. In certain cases, such as for example in the case of very large installations to make very large parts using high force, the complexity, size and cost of the motor, gearbox, and alignment coupling structures can be undesirably expensive, complex, large, and expensive to produce and repair. This can be true for example in making parts such as combustion casings and fan cases, or other parts, for large aircraft engines. Also, in some cases the original part to be rolled is imperfectly round, and so the rolling process transmits vibrations through the rollers back through the coupling and gearbox. The couplings and gearboxes thus must be selected or made to accommodate this transmitted vibration, which leads to further cost and complexity of these parts. Therefore it would desirable to have a rolling mill that improves at least to some extent in some applications, upon the disadvantages of electric motor driven rolling ring mills. The direct drive motor can include for example a hydraulic motor, a brushless DC motor (with a permanent magnet design with electronic switching) or a superconductor motor.